1. Field of the Invention
The present invention relates to a six-axis force sensor which can measure the forces in the directions of the X-, Y- and Z-axes and the moments around the X-, Y-, and Z-axes.
2. Description of the Related Art
For example, when an external force is applied to a front end of a robot arm, a load acts on a wrist of the arm. This load consists of forces in the directions of the X-, Y-, and Z-axes, and moments around the X-, Y-, and Z-axes. In order to measure such forces and moments, it is common to use a force sensor of the type which measures loads using the strain generated in a structure due to a load.
Japanese Unexamined Patent Publication No. 8-122178 discloses an example of such a force sensor. The six-axis force sensor disclosed in Japanese Unexamined Patent Publication No. 8-122178 includes a hollow cylindrical sensor body, and three or four legs built in the sensor body. Each leg includes a first arm and a second arm, which are disposed in directions substantially perpendicular to each other to thereby form an L-shape or T-shape. A first shear strain gauge is attached to the first arm so as to mainly detect a shear strain generated in a direction perpendicular to a longitudinal axial direction of the first arm within a plane where an L-shape or T-shape is formed in the first arm, and a second shear strain gauge is attached to the second arm so as to mainly detect a shear strain generated in a direction perpendicular to a longitudinal axial direction of the second arm within a plane where an L-shape or T-shape is formed in the second arm. Shear strains generated in the plane where an L-shape or T-shape is formed in each leg are detected by the first shear strain gauge and the second shear strain gauge, thereby measuring forces in the X-, Y-, and Z-axis directions and moments around the X-, Y-, and Z-axes, based on the detected shear strains.
The six-axis force sensor detects a shear strain generated on the surface of the leg forming an L-shape or T-shape. However, in the structure of the leg, a shear strain generated on the surface of the leg forming an L-shape or T-shape is smaller than an elongation strain generated by the warpage of each arm of the leg or the like. Thus, in order to improve sensitivity of the shear strain gauge to the shearing force, a concave pocket is provided on each of the first arm and the second arm, and each shear strain gauge is fitted into each pocket.
Presence of these pockets complicates a machining process for the leg and makes it difficult to provide a small robot. The shear strain gauge has a large size because it is required in principle to be capable of measuring a strain in two directions. Further, the shear strain gauge is more expensive than a single-axis-type strain gauge.